Fixes a crash which happens when trying to generate very large maps,
such as 256x256.
```
Exception of type `System.IndexOutOfRangeException`: Index was outside the bounds of the array.
at OpenRA.Mods.Common.MapGenerator.MatrixUtils.CalibrateQuantileInPlace(Matrix`1 matrix, Int32 target, Int32 count, Int32 outOf) in /home/ashley/devel/OpenRA/OpenRA.Mods.Common/MapGenerator/MatrixUtils.cs:line 719
at OpenRA.Mods.Common.Traits.RaMapGenerator.Generate(Map map, MiniYaml settings) in /home/ashley/devel/OpenRA/OpenRA.Mods.Common/Traits/World/RaMapGenerator.cs:line 755
...
```
19585ec7ac